scholarly journals Comparison of mean age of air in five reanalyses using the BASCOE transport model

2018 ◽  
Vol 18 (19) ◽  
pp. 14715-14735 ◽  
Author(s):  
Simon Chabrillat ◽  
Corinne Vigouroux ◽  
Yves Christophe ◽  
Andreas Engel ◽  
Quentin Errera ◽  
...  
Keyword(s):  
Climate Models ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Height Distribution ◽  
Heating Rates ◽  
Worst Case ◽  
Quasi Biennial Oscillation ◽  
Model Driven ◽  
Dipole Structure ◽  
Linear Trends

Abstract. We present a consistent intercomparison of the mean age of air (AoA) according to five modern reanalyses: the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim), the Japanese Meteorological Agency's Japanese 55-year Reanalysis (JRA-55), the National Centers for Environmental Prediction Climate Forecast System Reanalysis (CFSR) and the National Aeronautics and Space Administration's Modern Era Retrospective analysis for Research and Applications version 1 (MERRA) and version 2 (MERRA-2). The modeling tool is a kinematic transport model driven only by the surface pressure and wind fields. It is validated for ERA-I through a comparison with the AoA computed by another transport model. The five reanalyses deliver AoA which differs in the worst case by 1 year in the tropical lower stratosphere and more than 2 years in the upper stratosphere. At all latitudes and altitudes, MERRA-2 and MERRA provide the oldest values (∼5–6 years in midstratosphere at midlatitudes), while JRA-55 and CFSR provide the youngest values (∼4 years) and ERA-I delivers intermediate results. The spread of AoA at 50 hPa is as large as the spread obtained in a comparison of chemistry–climate models. The differences between tropical and midlatitude AoA are in better agreement except for MERRA-2. Compared with in situ observations, they indicate that the upwelling is too fast in the tropical lower stratosphere. The spread between the five simulations in the northern midlatitudes is as large as the observational uncertainties in a multidecadal time series of balloon observations, i.e., approximately 2 years. No global impact of the Pinatubo eruption can be found in our simulations of AoA, contrary to a recent study which used a diabatic transport model driven by ERA-I and JRA-55 winds and heating rates. The time variations are also analyzed through multiple linear regression analyses taking into account the seasonal cycles, the quasi-biennial oscillation and the linear trends over four time periods. The amplitudes of AoA seasonal variations in the lower stratosphere are significantly larger when using MERRA and MERRA-2 than with the other reanalyses. The linear trends of AoA using ERA-I confirm those found by earlier model studies, especially for the period 2002–2012, where the dipole structure of the latitude–height distribution (positive in the northern midstratosphere and negative in the southern midstratosphere) also matches trends derived from satellite observations of SF6. Yet the linear trends vary substantially depending on the considered period. Over 2002–2015, the ERA-I results still show a dipole structure with positive trends in the Northern Hemisphere reaching up to 0.3 yr dec−1. No reanalysis other than ERA-I finds any dipole structure of AoA trends. The signs of the trends depend strongly on the input reanalysis and on the considered period, with values above 10 hPa varying between approximately −0.4 and 0.4 yr dec−1. Using ERA-I and CFSR, the 2002–2015 trends are negative above 10 hPa, but using the three other reanalyses these trends are positive. Over the whole period (1989–2015) each reanalysis delivers opposite trends; i.e., AoA is mostly increasing with CFSR and ERA-I but mostly decreasing with MERRA, JRA-55 and MERRA-2. In view of this large disagreement, we urge great caution for studies aiming to assess AoA trends derived only from reanalysis winds. We briefly discuss some possible causes for the dependency of AoA on the input reanalysis and highlight the need for complementary intercomparisons using diabatic transport models.

scholarly journals Comparison of mean age of air in five reanalyses using the BASCOE transport model

2018 ◽  
Author(s):  
Simon Chabrillat ◽  
Corinne Vigouroux ◽  
Yves Christophe ◽  
Andreas Engel ◽  
Quentin Errera ◽  
...  
Keyword(s):  
Climate Models ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Height Distribution ◽  
Heating Rates ◽  
Worst Case ◽  
Model Driven ◽  
Model Studies ◽  
Dipole Structure ◽  
Linear Trends

Abstract. We present a consistent intercomparison of the mean Age of Air (AoA) according to five modern reanalyses: the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim), the Japanese Meteorological Agency’s Japanese 55-year Reanalysis (JRA-55), the National Centers for Environmental Prediction Climate Forecast System Reanalysis (CFSR) and the National Aeronautics and Space Administration’s Modern Era Retrospective-analysis for Research Applications version 1 (MERRA) and version 2 (MERRA-2). The modeling tool is a kinematic transport model driven only by the surface pressure and wind fields. It is validated for ERA-I through a comparison with the AoA computed by another transport model. The five reanalyses deliver AoA which differ in the worst case by one year in the tropical lower stratosphere and more than two years in the upper stratosphere. At all latitudes and altitudes, MERRA-2 and MERRA provide the oldest values (~ 5–6 years in mid-stratosphere at mid-latitudes) while JRA-55 and CFSR provide the youngest values (~ 4 years) and ERA-I delivers intermediate results. The spread of AoA at 50 hPa is as large as the spread obtained in a comparison of Chemistry-Climate Models. The differences between tropical and mid-latitudes AoA are in better agreement except for MERRA-2. Compared with in-situ observations, they indicate that the upwelling is too fast in the tropical lower stratosphere. The general hierarchy of reanalyses delivering older AoA (MERRA, MERRA-2) and younger AoA (JRA-55, CFSR) holds during the whole 1989–2015 period, with AoA derived from ERA-I keeping intermediate values. The spread between the five simulations in the northern mid-latitudes is as large as the observational uncertainties in a multidecadal time series of balloon observations, i.e., approximately two years. No global impact of the Pinatubo eruption can be found in our simulations of AoA, contrarily to a recent study which used a diabatic transport model driven by ERA-I and JRA-55 winds and heating rates. The time variations are also analyzed through multiple linear regression analyses taking into account the seasonal cycles, the Quasi-Biennal Oscillation and the linear trends over four time periods. The amplitudes of AoA seasonal variations in the lower stratosphere are significantly larger using MERRA and MERRA-2 than with the other reanalyses (up to twice as large at the 50 hPa pressure level). The linear trends of AoA using ERA-I confirm those found by earlier model studies, especially for the period 2002–2012 where the dipole structure of the latitude-height distribution (positive in the northern mid-stratosphere and negative in the southern mid-stratosphere) also matches trends derived from satellite observations of SF6. Yet the linear trends vary considerably depending on the considered period. Over 2002–2015 the ERA-I results still show a dipole structure but it is much less pronounced, with positive trends in the northern hemisphere remaining significant only in the polar lower stratosphere (where they reach 0.2 years per decade). No reanalysis other than ERA-I finds any dipole structure of AoA trends. The signs of the trends depend strongly on the input reanalysis and on the considered period, with values above 10 hPa varying between approximately −0.4 and 0.4 years per decade. Using ERA-I and CFSR, the 2002–2015 trends are negative above 10 hPa but using the three other reanalyses these trends are positive. Over the whole period 1989–2015 each reanalysis delivers opposite trends, i.e., AoA is mostly increasing with CFSR and ERA-I but mostly decreasing with MERRA, JRA-55 and MERRA-2. In view of these large disagreements, we urge great caution for studies aiming to assess AoA trends derived only from reanalysis winds. We briefly discuss some possible causes for the dependency of AoA on the input reanalysis and highlight the need for complementary intercomparisons using diabatic transport models.

scholarly journals The stratospheric Brewer–Dobson circulation inferred from age of air in the ERA5 reanalysis

2021 ◽  
Author(s):  
Felix Ploeger ◽  
Mohamadou Diallo ◽  
Edward Charlesworth ◽  
Paul Konopka ◽  
Bernard Legras ◽  
...  
Keyword(s):  
Diabatic Heating ◽  
Climate Models ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Residual Circulation ◽  
Heating Rates ◽  
Weather Forecasts ◽  
Trend Pattern ◽  
Transit Times ◽  
Medium Range

Abstract. This paper investigates the global stratospheric Brewer–Dobson circulation (BDC) in the ERA5 meteorological reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). The analysis is based on simulations of stratospheric mean age of air, including the full age spectrum, with the Lagrangian transport model CLaMS, driven by winds and total diabatic heating rates from the reanalysis. ERA5-based results are compared to those of the preceding ERA–Interim reanalysis. Our results show a significantly slower BDC for ERA5 than for ERA–Interim, manifesting in weaker diabatic heating rates and larger age of air. In the tropical lower stratosphere, heating rates are 30–40 % weaker in ERA5, likely correcting a known bias in ERA–Interim. Above, ERA5 age of air appears slightly high-biased and the BDC slightly slow compared to tracer observations. The age trend in ERA5 over 1989–2018 is negative throughout the stratosphere, as climate models predict in response to global warming. However, the age decrease is not linear over the period but exhibits steplike changes which could be caused by muti-annual variability or changes in the assimilation system. Over the 2002–2012 period, ERA5 age shows a similar hemispheric dipole trend pattern as ERA–Interim, with age increasing in the NH and decreasing in the SH. Shifts in the age spectrum peak and residual circulation transit times indicate that reanalysis differences in age are likely caused by differences in the residual circulation. In particular, the shallow BDC branch accelerates similarly in both reanalyses while the deep branch accelerates in ERA5 and decelerates in ERA–Interim.

scholarly journals Evaluation of the interactive stratospheric ozone (O3v2 module) for the E3SM version 2 Earth System Model

2020 ◽  
Author(s):  
Qi Tang ◽  
Michael J. Prather ◽  
Juno Hsu ◽  
Daniel J. Ruiz ◽  
Philip J. Cameron-Smith ◽  
...  
Keyword(s):  
Climate Models ◽  
Transport Model ◽  
Stratospheric Ozone ◽  
Earth System Model ◽  
System Model ◽  
Earth System ◽  
Heating Rates ◽  
Quasi Biennial Oscillation ◽  
Column Ozone ◽  
The University

Abstract. Stratospheric ozone affects climate directly as the predominant heat source in the stratosphere and indirectly through chemical feedbacks controlling other greenhouse gases. The U.S. Department of Energy's Energy Exascale Earth System Model version 1 (E3SMv1) implemented a new ozone chemistry module that improves the simulation of the sharp tropopause gradients, replacing a version based partly on long-term average climatologies that poorly represented heating rates in the lowermost stratosphere. The new O3v2 module extends seamlessly into the troposphere and preserves the naturally sharp cross-tropopause gradient, with 20-40% less ozone in this region. Additionally, O3v2 enables the diagnosis of stratosphere-troposphere exchange flux of ozone, a key budget term lacking in E3SMv1. Here, we evaluate key features in ozone abundance and other closely related quantities in atmosphere-only E3SMv1 simulations driven by observed sea surface temperatures (SSTs, years 1990-2014), comparing with satellite observations and the University of California, Irvine chemistry transport model (UCI CTM) using the same stratospheric chemistry scheme but driven by European Centre forecast fields for the same period. In terms of stratospheric column ozone, O3v2 shows improved mean bias and northern mid-latitude variability, but not quite as good as the UCI CTM. As expected, SST forcing does not match the observed quasi-biennial oscillation, which is mostly matched with the UCI CTM. This new O3v2 E3SM model retains mostly the same climate state and climate sensitivity as the previous version, and we recommend its use for other climate models that still use ozone climatologies.

scholarly journals Age of stratospheric air in the ERA-Interim

2012 ◽  
Vol 12 (24) ◽  
pp. 12133-12154 ◽  
Author(s):  
M. Diallo ◽  
B. Legras ◽  
A. Chédin
Keyword(s):  
Annual Cycle ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Maximum Amplitude ◽  
Heating Rates ◽  
Annual Modulation ◽  
Model Driven ◽  
The Mean ◽  
Mean Circulation ◽  
The Tropics

Abstract. The Brewer-Dobson mean circulation and its variability are investigated in the ERA-Interim over the period 1989-2010 by using an off-line Lagrangian transport model driven by analysed winds and heating rates. At low and mid-latitudes, the mean age of air in the lower stratosphere is in good agreement with ages derived from aircraft, high altitude balloon and satellite observations of long-lived tracers. At high latitude and in the upper stratosphere, we find, however that the ERA-Interim ages exhibit an old bias, typically of one to two years. The age spectrum exhibits a long tail except in the low tropical stratosphere which is modulated by the annual cycle of the tropical upwelling. The distribution of ages and its variability is consistent with the existence of two separate branches, shallow and deep, of the Brewer-Dobson circulation. Both branches are modulated by the tropical upwelling and the shallow branch is also modulated by the subtropical barrier. The variability of the mean age is analysed through a decomposition in terms of annual cycle, QBO, ENSO and trend. The annual modulation is the dominating signal in the lower stratosphere and is maximum at latitudes greater than 50° in both hemispheres with oldest ages at the end of the winter. The phase of the annual modulation is also reversed between below and above 25 km. The maximum amplitude of the QBO modulation is of about 0.5 yr and is mostly concentrated within the tropics between 25 and 35 km. It lags the QBO wind at 30 hPa by about 8 months. The ENSO signal is small and limited to the lower northen stratosphere. The age trend over the 1989–2010 period, according to this ERA-Interim dataset, is significant and negative, of the order of −0.3 to −0.5 yr dec−1, within the lower stratosphere in the Southern Hemisphere and south of 40° N in the Northern Hemisphere below 25 km. The age trend is positive (of the order of 0.3 yr dec−1) in the mid stratosphere but there is no region of consistent significance. This suggests that the shallow and deep Brewer-Dobson circulations may evolve in opposite directions. Finally, we find that the long lasting influence of the Pinatubo eruption can be seen on the age of air from June 1991 until the end of 1993 and can bias the statistics encompassing this period.

scholarly journals On the hiatus in the acceleration of tropical upwelling since the beginning of the 21st century

2014 ◽  
Vol 14 (23) ◽  
pp. 12803-12814 ◽  
Author(s):  
J. Aschmann ◽  
J. P. Burrows ◽  
C. Gebhardt ◽  
A. Rozanov ◽  
R. Hommel ◽  
...  
Keyword(s):  
21St Century ◽  
Structural Changes ◽  
Climate Models ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Early Period ◽  
Implicit Assumption ◽  
Heating Rates ◽  
Apparent Contradiction ◽  
High Bias

Abstract. Chemistry–climate models predict an acceleration of the upwelling branch of the Brewer–Dobson circulation as a consequence of increasing global surface temperatures, resulting from elevated levels of atmospheric greenhouse gases. The observed decrease of ozone in the tropical lower stratosphere during the last decades of the 20th century is consistent with the anticipated acceleration of upwelling. However, more recent satellite observations of ozone reveal that this decrease has unexpectedly stopped in the first decade of the 21st century, challenging the implicit assumption of a continuous acceleration of tropical upwelling. In this study we use three decades of chemistry-transport-model simulations (1980–2013) to investigate this phenomenon and resolve this apparent contradiction. Aside from a high-bias between 1985–1990, our model is able to reproduce the observed tropical lower stratosphere ozone record. A regression analysis identifies a significant decrease in the early period followed by a statistically robust trend-change after 2002, in qualitative agreement with the observations. We demonstrate that this trend-change is correlated with structural changes in the vertical transport, represented in the model by diabatic heating rates taken from the reanalysis product Era-Interim. These changes lead to a hiatus in the acceleration of tropical upwelling between 70–30 hPa and a southward shift of the tropical pipe at 30 and 100 hPa during the past decade, which appear to be the primary causes for the observed trend-change in ozone.

scholarly journals The stratospheric Brewer-Dobson circulation inferred from age of air in the ERA5 reanalysis

2021 ◽  
Author(s):  
Felix Ploeger ◽  
Mohamadou Diallo ◽  
Edward Charlesworth ◽  
Paul Konopka ◽  
Bernard Legras ◽  
...  
Keyword(s):  
Diabatic Heating ◽  
Climate Models ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Residual Circulation ◽  
Heating Rates ◽  
Weather Forecasts ◽  
Trend Pattern ◽  
Transit Times ◽  
Medium Range

<p>This paper investigates the global stratospheric Brewer-Dobson circulation (BDC) in the ERA5 meteorological reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). The analysis is based on simulations of stratospheric mean age of air, including the full age spectrum, with the Lagrangian transport model CLaMS, driven by winds and total diabatic heating rates from the reanalysis. ERA5-based results are compared to those of the preceding ERA-Interim reanalysis. Our results show a significantly slower BDC for ERA5 than for ERA-Interim, manifesting in weaker diabatic heating rates and larger age of air. In the tropical lower stratosphere, heating rates are 30-40% weaker in ERA5, likely correcting a known bias in ERA-Interim. Above, ERA5 age of air appears slightly high-biased and the BDC slightly slow compared to tracer observations. The age trend in ERA5 over 1989-2018 is negative throughout the stratosphere, as climate models predict in response to global warming. However, the age decrease is not linear over the period but exhibits steplike changes which could be caused by muti-annual variability or changes in the assimilation system. Over the 2002-2012 period, ERA5 age shows a similar hemispheric dipole trend pattern as ERA-Interim, with age increasing in the NH and decreasing in the SH. Shifts in the age spectrum peak and residual circulation transit times indicate that reanalysis differences in age are likely caused by differences in the residual circulation. In particular, the shallow BDC branch accelerates similarly in both reanalyses while the deep branch accelerates in ERA5 and decelerates in ERA-Interim.</p>

scholarly journals Modeling the transport of very short-lived substances into the tropical upper troposphere and lower stratosphere

2009 ◽  
Vol 9 (5) ◽  
pp. 18511-18543 ◽  
Author(s):  
J. Aschmann ◽  
B. M. Sinnhuber ◽  
E. L. Atlas ◽  
S. M. Schauffler
Keyword(s):  
Large Scale ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Three Dimensional ◽  
Chemical Transport Model ◽  
Heating Rates ◽  
Model Calculations ◽  
Upper Troposphere ◽  
Mass Fluxes ◽  
Tropical Upper Troposphere

Abstract. The transport of very short-lived substances into the tropical upper troposphere and lower stratosphere is investigated by a three-dimensional chemical transport model using archived convective updraft mass fluxes (or detrainment rates) from the European Centre for Medium-Range Weather Forecast's ERA-Interim reanalysis. Large-scale vertical velocities are calculated from diabatic heating rates. With this approach we explicitly model the large scale subsidence in the tropical troposphere with convection taking place in fast and isolated updraft events. The model calculations agree generally well with observations of bromoform and methyl iodide from aircraft campaigns and with ozone and water vapor from sonde and satellite observations. Using a simplified treatment of dehydration and bromine product gas washout we give a range of 1.6 to 3 ppt for the contribution of bromoform to stratospheric bromine, assuming a uniform source in the boundary layer of 1 ppt. We show that the most effective region for VSLS transport into the stratosphere is the West Pacific, accounting for about 55% of the bromine from bromoform transported into the stratosphere under the supposition of a uniformly distributed source.

scholarly journals On the hiatus in the acceleration of tropical upwelling since the beginning of the 21st century

2014 ◽  
Vol 14 (7) ◽  
pp. 9951-9973 ◽  
Author(s):  
J. Aschmann ◽  
J. P. Burrows ◽  
C. Gebhardt ◽  
A. Rozanov ◽  
R. Hommel ◽  
...  
Keyword(s):  
21St Century ◽  
Climate Models ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Early Period ◽  
Satellite Observations ◽  
Implicit Assumption ◽  
Apparent Contradiction ◽  
Chemistry Transport Model ◽  
Global Surface

Abstract. Chemistry–climate models predict an acceleration of the upwelling branch of the Brewer–Dobson circulation as a consequence of increasing global surface temperatures, resulting from elevated levels of atmospheric greenhouse gases. The observed decrease of ozone in the tropical lower stratosphere during the last decades of the 20th century is consistent with the anticipated acceleration of upwelling. However, more recent satellite observations of ozone reveal that this decrease has unexpectedly stopped in the first decade of the 21st century, challenging the implicit assumption of a continuous acceleration of tropical upwelling. In this study we use three decades of chemistry-transport-model simulations (1980–2013) to investigate this phenomenon and resolve this apparent contradiction. Our model reproduces the observed tropical lower stratosphere ozone record, showing a significant decrease in the early period followed by a statistically robust trend-change after 2002. We demonstrate that this trend-change is correlated with corresponding changes in the vertical transport and conclude that a hiatus in the acceleration of tropical upwelling occurred during the last decade.

scholarly journals Evaluation of the interactive stratospheric ozone (O3v2) module in the E3SM version 1 Earth system model

2021 ◽  
Vol 14 (3) ◽  
pp. 1219-1236
Author(s):  
Qi Tang ◽  
Michael J. Prather ◽  
Juno Hsu ◽  
Daniel J. Ruiz ◽  
Philip J. Cameron-Smith ◽  
...  
Keyword(s):  
Climate Models ◽  
Transport Model ◽  
Stratospheric Ozone ◽  
Earth System Model ◽  
System Model ◽  
Earth System ◽  
Heating Rates ◽  
Column Ozone ◽  
The University

Abstract. Stratospheric ozone affects climate directly as the predominant heat source in the stratosphere and indirectly through chemical reactions controlling other greenhouse gases. The U.S. Department of Energy's Energy Exascale Earth System Model version 1 (E3SMv1) implemented a new ozone chemistry module that improves the simulation of the sharp tropopause gradients, replacing a version based partly on long-term average climatologies that poorly represented heating rates in the lowermost stratosphere. The new O3v2 module extends seamlessly into the troposphere and preserves the naturally sharp cross-tropopause gradient, with 20 %–40 % less ozone in this region. Additionally, O3v2 enables the diagnosis of stratosphere–troposphere exchange flux of ozone, a key budget term lacking in E3SMv1. Here, we evaluate key features in ozone abundance and other closely related quantities in atmosphere-only E3SMv1 simulations driven by observed sea surface temperatures (SSTs, years 1990–2014), comparing them with satellite observations of ozone and also with the University of California, Irvine chemistry transport model (UCI CTM) using the same stratospheric chemistry scheme but driven by European Centre forecast fields for the same period. In terms of stratospheric column ozone, O3v2 shows reduced mean bias and improved northern midlatitude variability, but it is not quite as good as the UCI CTM. As expected, SST-forced E3SMv1 simulations cannot synchronize with observed quasi-biennial oscillations (QBOs), but they do show the typical QBO pattern seen in column ozone. This new O3v2 E3SMv1 model mostly retains the same climate state and climate sensitivity as the previous version, and we recommend its use for other climate models that still use ozone climatologies.

scholarly journals Age of stratospheric air in the ERA-Interim

2012 ◽  
Vol 12 (7) ◽  
pp. 17087-17134 ◽  
Author(s):  
M. Diallo ◽  
B. Legras ◽  
A. Chedin
Keyword(s):  
High Altitude ◽  
Southern Hemisphere ◽  
Northern Hemisphere ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Maximum Amplitude ◽  
Positive Trend ◽  
Heating Rates ◽  
Annual Modulation ◽  
The Mean

Abstract. The age of stratospheric air is calculated over 22 yr of the ERA-Interim reanalysis using an off-line Lagrangian transport model and heating rates. At low and mid-latitudes, the mean age of air is in good agreement with observed ages from aircraft flights, high altitude balloons and satellite observations of CO2 and SF6. The mid-latitude age spectrum in the lower stratosphere exhibits a long tail with a peak at 0.5 yr, which is maximum at the end of the winter, and a secondary flat maximum between 4 and 5 yr due to the combination of fast and slow branches of the Brewer-Dobson circulation and the reinforced barrier effect of the jet. At higher altitudes, the age spectrum exhibits the footprint of the annual modulation of the deep Brewer-Dobson circulation. The variability of the mean age is analysed through a decomposition in terms of annual cycle, QBO, ENSO and trend. The annual modulation is the dominating signal in the lower stratosphere and in the tropical pipe with amplitude up to one year. The phase of the oscillation is opposite in both hemisphere beyond 20° and is also reversed below and above 25 km with maximun arising in mid-March in the Northern Hemisphere and in mid-September in the Southern Hemisphere. The tropical pipe signal is in phase with the lower southern stratosphere and the mid northern stratosphere. The maximum amplitude of the QBO modulation is of about 0.5 yr and is mostly concentrated within the tropics between 25 and 35 km. It lags the QBO wind at 30 hPa by about 8 months. The ENSO signal is small and limited to the lower northen stratosphere. The trend is significant and negative, of the order of −0.3 to −0.5 yr dec−1, within the lower stratosphere in the Southern Hemisphere and under 40° N in the Northern Hemisphere below 25 km. It is positive (of the order of 0.3 yr dec−1) in the mid stratosphere but there is no region of consistent significance. This suggests that the shallow and deep Brewer-Dobson circulations may evolve in opposite directions. It is however difficult to estimate a reliable long-term trend from only 22 yr of data. For instance, a positive trend is found in the lower stratosphere if only the second half of the period is considered in agreement with MIPAS SF6 data excepted in the northern polar region and at high altitude. Finally, it is found that the long lasting influence of the Pinatubo eruption can be seen on the age of air from June 1991 until the end of 1993 and can bias the statistics encompassing this period. In our analysis, this eruption shifts the trend towards negative values by about 0.2 to 0.3 yr dec−1.

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